Computer disc drives store information on rotating recordable discs. The information is stored in concentric tracks on the recording disc. A transducer element which flies on a thin film of air over the surface of the disc is moved from track to track on these discs to read and record the desired information. Typically, the rotating discs are closely held apart in a parallel stacked relationship. Sometimes however, only one rotating disc is found in the disc drive. In either case, in order to achieve a tight packing density of bits on the disc, the transducer elements must be held very closely spaced from the rotating disc surface.
As a result, as track densities and drive speeds increase, disc vibration becomes a significant cause of track misregistration. The rigid discs for disc drives need to have good mechanical properties in terms of stiffness, as it has been found that "out of plane" vibrations or "flutter" during high RPM operation can lead to this track misregistration. According to an article by McAllister in "Data Storage" May-June, 1997, pp. 29-32, the vibration or flutter is primarily caused by internal air flow disturbances. It has been found that one way to lower vibration amplitude is to increase the substrate stiffness as measured by the elastic modulus E, as reported in "Characterization of Disc Vibrations on Aluminum and Alternate Substrates", by McAllister, IEEE Trans. Magn, Vol. 33, pp. 968 (1997). According to McAllister, another way to decrease vibration amplitude is to increase substrate damping. In this category, one scheme has been proposed to make a laminated structure by providing a sandwiched of a thin visco-elastic film between two metallic layers, which would typically be aluminum.
However, with the advent of another technology, i.e., the plastic disc substrate, other solutions must be found. This technology is used widely in removable storage applications such as CD-ROM and magneto-optical; the technology is extremely appealing for rigid disc drives as it offers an extremely inexpensive and simple disc process (injection molding). Such a process, in principle, would not require any surface finishing (grinding, polishing) and washing. However, most plastic substrates are likely to offer unacceptable vibration characteristics at high RPM if made thin (31.5 mils or thinner).